Modern liquid crystal displays (LCDs) require radiant color, fast-response time, high contrast, and wide-viewing angle properties as well as high reflectivity for low-power-consumption devices. Compensation films are a vital component for many high performance liquid crystal displays. Specifically, compensation films are designed to make the optical symmetry of the compensators more closely resemble that of the director distribution in the liquid crystal displays. Wide range of compensation films, with optical axis oriented in-plane, titled, or normal to the plane of the film have been developed as achromatic films for STN-LCD and recently, been applied to improve the viewing angle characteristic of TFT-LCD. The optical properties of the retardation films are getting more and more important with an increase in the application field such as reflective LCDs, in which compensation films are used as a quarter wave film to improve the display contrast. It is further envisioned that these applications will expand even further as such films become more readily available and also in increasing sizes.
Conventional optical compensation films are produced from polymer materials such as, stretched polymer films, cast polymer films, or photopolymerized anisotropic polymer films. Stretched polycarbonate quarter wave films or cast polymer films are well-known to have a problem in wavelength dispersion of optical retardation. In practical applications, for example, a film-compensated, normally-white reflective TN at ON state leads to a bluish appearance and the deterioration of optical contrast. Two methods to improve the dispersion property have been proposed: one is the method of combining one or two half wave films with a quarter wave film and the other one is to use special cellulose derivatives as film materials. However, the former approach results in an increase in thickness of the retardation film and to the complexity of the manufacturing process. The latter method still has not yet totally solved the wavelength dispersion problem of retardation compared with that of former method.
Liquid crystal polymer retardation films have been developed for the viewing angle improvement of LCDs. These films are produced either by solution-casting from liquid crystal polymers dissolved in appropriate solvents or photo-polymerizing thin films of rod-like or disc-like liquid crystal monomers on substrates which provide the alignment for the mesogenic monomers, i.e. monomers which contain a mesogenic moiety and one or more polymerizable groups such as, acrylate(s), methacrylate(s), epoxide(s), thio-ene(s), vinyl ether(s), and oxetne(s). The wavelength dispersion property of these anisotropic retardation films is rarely discussed, especially on those successful low wavelength dispersion compensation films. There is a need for a method of fabricating an optical compensation film without the need for stretching, or applying electric or magnetic field which complicates the processing feasibility. Further, the method needs to be capable of fabricating large-sized films with high yield, such as meter-wide, roll-to-roll continuous process films.
Polymer stabilization is a unique technique to stabilize the molecular conformation by polymer network in which a heterogenous phase separated composite is formed with a matrix of the stabilization polymer and discrete domains of the guest liquid crystal, or vice verse, depending on the relative ratio of the two phases. In general, the polymer network is produced by photo-polymerization of an appropriate amount of reactive molecules in a liquid crystal phase. The amount selected may depend on the processing parameters which are selected, but generally will be from about 3 to about 99%, more preferably about 5 to about 95%, or about 5 to about 80%, or even about 60% by weight of the reactive monomer(s) with the weight percent remainder being the liquid crystal, and the percentages based on weight percent. This technique has been successfully applied to produce LC/polymer composites which are believed to primarily be phase separated, heterogenous composites, with unique structures or optical properties. Further this technique has led to the development of various displays such as, light scattering LCDs, bi-stable reflective cholesteric displays, and threshold-less ferroelectric displays. “Bi-stable” is used here in its generally accepted meaning, i.e. having two different optical states such as a dark and a bright state which are stable without requiring an externally applied field.